The facts: 10" blocks... the instructions say to quilt a big "X" across each block... an experienced quilter at a favorite LQS suggested I might quilt in the ditch prior to the criss-cross quilting, and I like that idea.
The question: I know I should start in the middle of the quilt and work my way out. I'm thinking I should change directions every row. Should I quilt all of the horizontal ditches and THEN the vertical ditches... OR... should I quilt 3-5 horizontal ditches, then some vertical, then horizontal... you get the picture...
OR... does it matter?
Please help? I'm very excited to start quilting for the FIRST time!!
THANKS in advance to all who help!!

As long as you have pinned it properly with the back as taught as you can get it, it shouldn't matter if you do all the vertical lines first, then the horizontal.
Just make sure you check the back each time and keep it tight when stitching. You might have to pull it as you go.
It is smart to always start in the middle and make your way to the sides.
Go for it. Practice is a good teacher.

I would recommend starting in the middle of a top/bottom and doing all the vertical lines first, but starting from the middle working out to both sides. Then do all of the horizontal (side-to-side lines), again starting in the middle and working out to the ends. I recommend doing the long lines first because then, when you go to do the shorter lines, you won't have so many lines to cross (and create potential puckers).

I do not think it is necessary to change direction with every line but, if that is the easiest way to manipulate the quilt sandwich it should be fine.

Have you create the quilt sandwich yet? If not, I recommend that you starch the backing fabric heavily before layering, and also spray starch the top. Starching will stabilize the fabrics so you will be less likely to get puckers.

If the sandwich is already made, I recommend laying out a large sheet to catch overspray and spray starching both sides several times. (Spray, wait to dry, spray again, etc. before turning and doing the other side.) Again, the starch will help stabilize the fabrics.

If at all possible, use a walking foot for the lines. Your greatest problem will be puckers on the back where your quilting line meets a previous quilting line. Using a walking foot and starching heavily will help minimize this problem.

I do change directions with each row, but notice that some others don't find that necessary. Whatever works best for you. Just be sure to check for puckers after each line.

I also start with the longer lines, center to sides. I change direction with each row because once I didn't and it creeped around too much for my liking. If the quilt will get a lot of use, consider using a decorative stitch over the ditch instead of SID. Also, if it is for a young person, try quilting shapes (circles, triangles, squares ... ) in the blocks instead of simply an X.